This invention relates to the preparation of the surface of indium phosphide, and, more particularly, to such preparation including the chemical toughening of the surface.
In one approach to manufacturing a microelectronic device, electronic components are deposited upon the front side of a thin wafer of a substrate semiconductor material such as indium phosphide (InP). The wafer of indium phosphide is relatively thick, about 0.025 inches, at this deposition stage of the fabrication procedure. With the deposited electronic components in place on the front side of the wafer, the wafer is lapped from the back side to thin it to a final thickness of about 0.004 inches. Openings or vias are formed through the wafer from the back side to the electronic circuitry on the front side. A metallic layer is deposited upon the back side and patterned with external connection points and internal interconnects, and external connections are made to the metallic layer.
It is important that the metallic layer have good adhesion to the back side of the wafer, so that it will not separate and flake or peel away during service. If that were to happen, the microelectronic device might become inoperable. A problem arises because, during the fabrication process Just described, the metallic layer must be deposited upon the back side surface that has just been lapped. Lapping is essentially a polishing process to remove the excess thickness of semiconductor material, and in the lapping process the back side of the wafer becomes very smooth and shiny. A deposited layer of metal does not adhere ("stick") well to such a smooth, shiny surface, and is prone to debonding during fabrication or service.
It is known that the metallic layer will have improved adherence to a somewhat rougher (i.e., less smooth) back side surface structure than that produced by the lapping operation. To improve the adhesion of the metallic layer to the back side of the semiconductor wafer, various mechanical roughening techniques have been tried. For example, sandblasting or sanding the lapped surface with coarse sandpaper can produce a controllably rough back side surface to which the metallic layer adheres well. However, these mechanical roughening techniques also damage the wafer by introducing surface imperfections such as fine microcracks into the back surface of the wafer. These imperfections can ultimately cause structural failure of the wafer initiated at the damaged surface during service.
A better approach to fabrication of these microelectronic devices utilizing metallic coating of the back side of a lapped semiconductor wafer is therefore required. Such an approach must be compatible with the presence of the electronic circuit elements on the front side, and must ensure good adherence of the metallic coating to the back side of the wafer. The present invention fulfills this need, and further provides related advantages.